Laser-based wind sensor lets turbines grab more power

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A sensor that can see the wind ahead of a turbine would let it react to changes in wind direction and speed, thereby generate more power.

For decades, large utility scale wind turbines have been forced to rely on mechanical wind sensing devices that are mounted on the nacelles of the turbines to measure the wind speed and direction. These signals are fed into the wind turbine control system to adjust the turbine nacelle to account for the variations in the wind flow. The trouble with these devices is that they are located in the wrong position, in extremely turbulent airflows that must be time averaged to smooth out the fluctuations caused by blade interference and turbulence from the aerodynamic flow over the nacelle. The wind measured behind the wind turbine rotor blades bears no resemblance to the actual airflow that the turbine rotor system encounters flowing into the blades. The resulting misalignment in yaw and pitch, caused by the reactive nature of current wind sensors, has a tremendous affect on the efficiency of the wind turbine and the stress load damage imparted to the dynamic components.

Start with yaw

An important factor affecting wind-turbine efficiency is yaw alignment, a measure of a turbine rotor’s misalignment with the wind. To operate most efficiently, wind turbines should keep the turbine nacelle aligned with the wind. Because the wind speed and direction are always changing, the yaw misalignment is always present even when the winds appear to be “steady” as reported by the conventional mechanical or sonic anemometry in use. Although the alignment error between the wind direction and the turbine nacelle cannot be completely eliminated, its magnitude can be greatly reduced, producing more energy and reducing turbine stress loads. If a turbine’s control system can assess accurate wind direction and speed far enough in advance of the turbine, it can better align itself with approaching wind.

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Staying aligned with the wind significantly increases power output and reduces turbine stress. It also has the potential to guide the adjustment of blade pitch in anticipation of sudden stress events caused by gusts.

Catch the Wind, Inc. (CTW) has developed a laser wind sensor that, when mounted on the turbine nacelle, accurately measures real-time horizontal and vertical wind speed and direction. The Vindicator laser wind sensor (LWS) can look out to 300 meters ahead of the turbine to measure wind speed and direction as it approaches the turbine blades and transmits that data to the controls in sufficient time (20 seconds of lead time for a 35 mph wind) to reorient the turbine. The system is comprised of a base laser unit and a remote lens unit. The base unit is housed in a separate assembly that can be mounted inside the turbine’s nacelle and connected to the remote lens assembly. The technology uses Laser Doppler Velocimetry, an optical remote sensing technique similar to Doppler radar, to measure the minute frequency changes of light reflected by microscopic air particles moving with the wind to precisely determine wind speed and direction.

Challenges for mechanical sensors

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The Nordex turbine shows the relative location of the remote laser unit on the nacelle.

Today’s turbines operate reactively because the sensors are mounted on the back of the nacelle – behind the turbine rotor blades. That means after a wind change passes a turbine, its mechanical, or ultra-sonic sensors, must detect the change, average out the variations in the disturbed windflow and then provide control signals to adjust the turbine’s direction. The resultant averaging and time delay, often several minutes long, means that the wind turbine is not operating as effectively in capturing all the energy from the wind, particularly in Region 2 (between the turbines cut-in wind speed and rated power wind speed) where most wind turbines spend the bulk of their operating time. This near-constant misalignment also generates unnecessary stress on blades, causing premature wear and damage to them and key turbine components. Repair or replacement of these major components represents a significant cost to the wind farm operations, and when coupled to the out of service time for repair, directly impacts the profitability and cost of wind energy. A feed-forward, laser-based wind sensor addresses these issues by taking a proactive approach to wind turbine control by assessing the wind before it reaches the turbine.

Small changes, big payoff

Misaligned turbines result in lost revenue, either by power output losses due to inefficiency, or by increased maintenance costs due to stress damage. Because the power output is theoretically reduced by the cosine cubed of yaw angle, the apparent power losses start to add up rapidly when the turbine is misaligned by more than 10 degrees. Trial data from Catch the Wind’s testing in on an operating wind turbine in Nebraska demonstrates that the nacelle misalignment may be significantly greater than conventional industry understanding.

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The Nebraska Public Power District (NPPD) tested a Vindicator LWS last year on a Vestas V-82 wind turbine. The remote laser unit was mounted atop the nacelle and directed forward through the blades. The base laser unit was mounted inside the nacelle. A conservative control algorithm was implemented resulting in the Vindicator sensor controlling the turbine approximately half the time. During the month long test, the turbine’s power production showed an average increase of over 10% when the LWS was in control of the turbine yaw system. Additionally, a SWANTech Stress Wave Energy monitoring system recorded a significant decrease in stress loading on the main shaft bearing with the Vindicator system in control. With better control algorithms, the average power output figure could be improved even more. Since the initial one month trial, the Vindicator LWS has continued to demonstrate more than 10% average power output improvement over an additional seven months of data taken at NPPD.

By one estimate, there are more than 90,000 wind turbines worldwide rated at 1.5 MW and more. With increased emphasis of government mandates and industry targets for more renewable energy resources, that number is expected to more than double by the end of 2014. By 2020, demand for wind turbines of 1.5 MW capacity or greater is expected to exceed 500,000. If wind power is ever to realize its power generation potential, wind farm operators must consider more options to boost power production and revenues while lowering operational costs.

The future of laser wind sensors

Feed-forward laser-based devices will be increasingly used worldwide to measure the wind and provide more precise information for wind farms. Another important application for feed-forward laser sensors is to measure the gust and turbulence characteristics of the wind to proactively adjust blade pitch to capture additional wind energy sustained in the gusts and, if necessary, initiate control measures to prevent or reduce blade damage from increased turbulence.

Laser-based wind sensors will also improve a variety of critical wind-related activities, including wind resource assessment, turbine performance monitoring, wind prospecting, and real-time inputs for wind forecasting models used in grid balancing.

Grid managers will be able to increasingly rely on clean, renewable wind power for a greater percentage of the total energy demand instead of requiring higher reserves of carbon-based fuels such as gas or coal to account for the wind’s variability.

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Laser Doppler Velocimetry has also been adapted to monitoring wind conditions at sea. The wind roses to the right tell of different speed and directions with altitude.

For the maritime sector, Catch the Wind has partnered with AXYS Technologies to integrate the laser wind sensing technology into its WindSentinel product, the world’s first offshore wind resource assessment buoy. It accurately measures the wind speed profile up to the top of the rotor diameter of even the tallest wind turbine, providing offshore wind farm developers with a portable and reusable instrument to determine wind resources at potential wind farm sites.

The American Wind Energy Association predicts wind power could provide as much as 20% of the U.S.’s electricity needs by 2030. Should wind power reach that threshold, it will supply enough energy to displace about 50% of electric utility natural gas consumption by 2030, which will amount to an 11% reduction in natural gas across all industries. Additionally, coal consumption will be reduced by 18%. WPE

Comments

  1. Which do you think are the benefits of using two wind ultrasonic sensors together in a wind turbine?.
    Lately besides of the use or this new kind of instruments, the use of one pair of them it’s becoming more common.
    is it possible to use both signal to avoid flow disturbances?. is this way of sensing some kind of signal filtering?

  2. Paul Dvorak says:

    Mr. Smith:

    Thanks for your comments. You touch upon the most enlightening aspect of the wind industry: It underscores how much we don’t know about nature and mechanical structures, which means we have a lot to learn, which means the equipment can be improved greatly, and that’s good.

    As you suggest, conventional anemometers sit on the “back” of turbine nacelles, so they are telling the turbine controls what the wind was doing. The laser wind sensor looks ahead to the incoming wind and says, get ready for something different. Big difference. The wind changes a lot and does not always blow straight and level. The report in the story out of Nebraska (I think) showed a 10% gain in power production from a conservative control algorithm. The author suggested that figure could easily be improved upon, and that is good news for the wind industry.

    Paul Dvorak

  3. Thomas Code says:

    It increases production/efficiency while decreasing stress on components. The NPPD tests have supported this with 8 months of data. Very interesting and informative, it appears being pro-active instead of post-active is better.

  4. keropz smith says:

    How does this help a wind turbine? At the moment, wind turbines estimate the wind speed based on a nacelle-based anemometer – typically ultrasonic, or a traditional cup anemometers. The wind speed measured here is used as the basis for the controller to estimate the wind speed over the entire rotor and set the appropriate pitch setting of the blades to optimise the angle of attack.

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